The purpose of this study is to identify the changes, caused by human opiate addiction, in chromosomal structure and in DNA repair capacity which may have mutagenic, carcinogenic or pathological effects. The results obtained thus far demonstrate that opiate addiction reduced the capacity of peripheral lymphocytes to repair far ultraviolet light (254 nm) induced DNA damage, increased levels of chromosome aberrations and altered sister chromatid exchange response patterns. The effects of heroin on DNA repair and chromosome aberrations are ameliorated by long term methadone treatment. These findings indicate that opiate abusers are at high risk for genetic alteration, but they must be expanded and corroborated by other assays before their full significance can be explicated.